A mobile communication system has been developed as a high-speed and high-quality wireless packet data communication system in order to provide data services and multimedia services as well as voice-based services. In recent years, a variety of mobile communication standards such as High Speed Downlink Packet Access (HSDPA), High Speed Uplink Packet Access (HSUPA), Long Term Evolution (LTE), and Long Term Evolution Advanced (LTE-A) of the 3rd Generation Partnership Project (3GPP), High Rate Packet Data (HRPD) of the 3GPP2, and Institute of Electrical and Electronics Engineers (IEEE) 802.16 have been developed to support a high-speed and high-quality wireless packet data transmission service. In particular, the LTE system is a system that has been developed to efficiently support high-speed wireless packet data transmission, and may maximize a capacity of a wireless system using a variety of wireless access technologies. In addition, an LTE-A system, which evolved from the LTE system, has an improved data transmission capability when compared to the LTE system.
In general, the LTE system refers to a base station and a terminal corresponding to Release 8 or 9 of the 3GPP standard group, and the LTE-A system refers to a base station and a terminal corresponding to Release 10 of the 3GPP standard group. In the 3GPP standard group, the standardization of the subsequent Release having improved performance based on the standardization of the LTE-A system is ongoing even after the standardization of the LTE-A system.
The 3G and 4G wireless packet data communication system such as HSDPA, HSUPA, HRPD, LTE/LTE-A, etc. may use techniques such as an Adaptive Modulation and Coding (hereinafter, referred to as “Adaptive Modulation and Coding (AMC)”) method, a channel-sensitive scheduling method, and the like in order to improve the transmission efficiency. When using the AMC method, a transmitter may adjust an amount of data to be transmitted according to a channel status. By way of example, when the channel status is poor, the transmitter may adjust a reception error probability to a desired level by reducing the amount of data to be transmitted, and when the channel status is good, the transmitter may efficiently transmit a large amount of information while adjusting the reception error probability to the desired level by increasing the amount of data to be transmitted. When using a channel-sensitive scheduling resource management method, the transmitter may selectively serve a user having an excellent channel status among multiple users, and therefore the system capacity is increased when compared to allocating a channel to one user and serving the user. This capacity increase is often referred to as “Multi-user Diversity gain”. The AMC method and the channel-sensitive scheduling method may receive feedbacks of partial channel status information from a receiver, and apply an appropriate modulation and coding scheme at the time that is determined to be the most efficient.
When used together with a Multiple Input Multiple Output (MIMO) transmission method, the AMC method may include a function of determining the number of spatial layers of signals to be transmitted or a rank thereof. In this case, the AMC method may consider the number of layers to which data is transmitted using MIMO, without simply considering only the coding rate and the modulation scheme in determining the optimum data rate.
In recent years, research has been actively conducted to switch Code Division Multiple Access (CDMA), i.e., a multiple access scheme that has been used in the 2G and 3G mobile communication systems, to Orthogonal Frequency Division Multiple Access (OFDMA) in the next-generation communication system, and, in 3GPP and 3GPP2, the standardization on the evolved systems that use OFDMA is ongoing. It is well known that OFDMA may contribute to an increase in capacity, when compared to CDMA. By way of example, OFDMA can perform frequency domain scheduling on a frequency axis to thereby increase the capacity. As the capacity gain was obtained by the channel-sensitive scheduling method based on the time-varying characteristics of channels, additional capacity gain can be obtained by utilizing the frequency-dependent characteristics of channels.
FIG. 1 is a diagram illustrating time and frequency resources which are used in an LTE/LTE-A system according to the related art.
Referring to FIG. 1, wireless resources, which are allocated to user equipment (UE) by an enhanced Node B (eNB), are divided in a resource block (RB) unit on a frequency axis, and in a subframe unit on a time axis. Generally, one RB includes 12 subcarriers in the LTE/LTE-A system, and occupies a band of 180 kHz. Generally, one subframe includes 14 OFDM symbol sections in the LTE/LET-A system, and occupies a time section of 1 msec. When performing scheduling, the LTE/LTE-A system may allocate resources in the subframe unit on the time axis, and in the RB unit on the frequency axis.
FIG. 2 is a diagram illustrating wireless resources of one subframe and one RB which is a minimum unit for downlink scheduling in an LTE/LTE-A system according to the related art.
Referring to FIG. 2, the wireless resources may be configured with one subframe on the time axis, and one RB on the frequency axis. In this instance, one RB is constituted of 12 subcarriers in a frequency domain, and one subframe is constituted of 14 OFDM symbols in a time domain, so that the wireless resources have a total of 168 unique frequencies and time positions. In LTE/LTE-A, each unique frequency and time position of FIG. 2 is called a resource element (RE).
A plurality of different kinds of signals can be transmitted to the wireless resources illustrated in FIG. 2.
1) Cell Specific Reference Signal (CRS): Reference signal transmitted for all UEs belonging to one cell.
2) Demodulation Reference Signal (DMRS): Reference signal transmitted for a specific UE.
3) Physical Downlink Shared Channel (PDSCH): Data channel transmitted by a downlink, used in order for an eNB to transmit traffic to a UE, and transmitted using an RE in which a reference signal is not transmitted in data region of FIG. 2.
4) Channel Status Information Reference Signal (CSI-RS): Reference signal transmitted for UEs belonging to one cell, and used to measure channel status. Here, a plurality of CSI-RSs can be transmitted to one cell.
5) Other control channels (PHICH, PCFICH, and PDCCH): Provide control information required for receiving PDSCH by UE, or transmitting Acknowledgement/Negative Acknowledgement (ACK/NACK) for operating hybrid automatic repeat request (HARQ) concerning uplink data transmission.
Other than the above-described signals, the LTE-A system may set muting so that a CSI-RS transmitted by another eNB can be transmitted to UEs of the corresponding cell without interference. Muting may be applied in a position in which the CSI-RS can be transmitted, and the UE may receive a traffic signal over the corresponding wireless resources. Due to characteristics of muting, muting may be applied in the position of the CSI-RS and transmission power may not be transmitted, and therefore muting in the LTE-A system may be also referred to as zero-power CSI-RS.
In FIG. 2, the CSI-RS may be transmitted using a part of positions indicated by A, B, C, D, E, E, F, G, H, I, and J depending on the number of remote radio heads (RRHs) that transmit the CSI-RS, and muting may be also applied to the part of the positions indicated by A, B, C, D, E, E, F, G, H, I, and J. In particular, the CSI-RS may be transmitted to 2, 4, and 8 REs depending on the number of CSI-RS ports that transmit the CSI-RS. When the number of CSI-RS ports is 2, the CSI-RS may be transmitted to half of a specific pattern in FIG. 2, when the number of CSI-RS ports is 4, the CSI-RS may be transmitted to the whole of the specific pattern, and when the number of CSI-RS ports is 8, the CSI-RS may be transmitted using two patterns. On the other hand, muting may be always performed in a unit of one pattern. That is, muting may be applied to a plurality of patterns, but may not be applied to only a part of one pattern when a position of muting is not overlapped with the position of the CSI-RS. However, only when the position of the CSI-RS and the position of muting are overlapped with each other, muting may be applied to only a part of one pattern.
In order to measure a downlink channel status in a cellular system, a reference signal should be transmitted. In the LTE-A system of 3GPP, a UE measures a channel status between an eNB and the UE using a CSI-RS transmitted by the eNB. For the channel status, several factors such as an amount of interference in a downlink may be basically considered. The amount of interference in the downlink may include interference signals, thermal noise, and the like generated by RRHs belonging to a neighboring eNB, and may be important when the UE determines the downlink channel status. By way of example, when an eNB with one transmission RRH transmits data to a UE with one reception RRH, the UE should decide energy per symbol to interference density ratio (Es/Io) by determining energy per symbol that can be received by a downlink and an amount of interference that can be simultaneously received in a section that receives the corresponding symbol, from the reference signal received from the eNB. The decided Es/Io may be notified to the eNB, and the eNB may decide a data transmission rate of the downlink that transmits data to the UE using the Es/Io.
In the case of a general mobile communication system, an eNB equipment is disposed in the midpoint of each cell, and the corresponding eNB equipment performs mobile communication with the UE using one or a plurality of RRHs as located in a limited position. A mobile communication system in which RRHs belonging to a single cell are arranged in the same location in this manner is referred to as a centralized antenna system (CAS). On the other hand, a mobile communication system in which RRHs belonging to a single cell are arranged in distributed locations within the cell is referred to as a distributed antenna system (DAS).
FIG. 3 is a diagram illustrating distributed locations of RRHs arranged in a general DAS according to the related art.
Referring to FIG. 3, a DAS includes two cells 300 and 310, and each of the cells 300 and 310 includes a single high-output RRH 320 and four low-output RRHs 340. In this instance, the high-output RRH 320 may provide minimal service throughout the entire region included in a cell region, and the low-output RRHs 340 may provide a high data rate-based service to limited UEs in a limited region within the cell. In addition, the low-output RRHs 340 and the high-output RRH 320 are all connected to a central controller (not illustrated) as shown by dotted lines 330, and thereby operated according to scheduling and wireless resource allocation of the central controller. In a single RRH location geographically separated from the DAS, one or a plurality of RRHs may be arranged. One or a plurality of RRHs arranged in the same location in the DAS may be referred to as an RRH group.
In the DAS shown in FIG. 3, a UE may receive signals from a single geographically separated RRH group, but signals transmitted from the remaining RRH groups may act as interference to the UE.
FIG. 4 is a diagram illustrating an interference phenomenon that occurs when data is transmitted to other UEs for each RRH group in a DAS according to the related art.
Referring to FIG. 4, a UE 1 (400), a UE 2 (420), a UE 3 (440), and a UE 4 (460) receive traffic signals from RRH groups 410, 430, 450, and 470, respectively. The UE 1 (400) may receive interference from other RRH groups that transmit the traffic signals to other UEs while receiving the traffic signals from the RRH group 410, and by way of example, in FIG. 4, signals transmitted from the RRH groups 430, 450, and 470 may generate an interference effect in the UE 1 (400).
In general, there are the following two kinds of interference that occur by another RRH group in a DAS.
1) Inter-cell interference: Interference that occurs in RRH group of other cell
2) Intra-cell interference: Interference that occurs in RRH group of the same cell
In FIG. 4, as intra-cell interference to the UE 1 (400), there is interference that occurs in the RRH group 430 belonging to the same cell, and as inter-cell interference, there is interference that occurs in the RRH groups 450 and 470 of a neighboring cell. The inter-cell interference and intra-cell interference may be received simultaneously with data channel signals of a UE to disturb reception of the data channel signals of the UE.
In order for a UE of the DAS to receive signals at an optimum data transmission rate using a downlink, it is necessary to accurately measure inter-cell interference and intra-cell interference that cause interference to the UE itself, and request a data transmission rate from an eNB based on a compared result between the measured interference and the strength of the received signals.
In the case of a general CAS rather than the DAS, there is only one RRH group for each cell. In this case, intra-cell interference that occurs between other RRH groups within the same cell as shown in FIG. 4 does not occur, and only inter-cell interference that occurs in other cells occurs. When the LTE/LTE-A system is configured as a CAS, it is possible to measure inter-cell interference using the CRS described in FIG. 2. In general, in the CAS, a UE receives the CRS, and then converts the CRS having periodic characteristics in a frequency domain into a delay domain signal using Inverse Fast Fourier Transform (IFFT).
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.